Complete Guide to Plastic Film Washing Lines: Comprehensive Solutions from Shredding to Drying

Plastic film washing lines are a crucial component in the plastic recycling process. They consist of a series of equipment designed to clean dirty films (such as LDPE films and woven bags) through friction and high-speed action, transforming them into clean raw materials suitable for pelletizing. These cleaned film granules can then be used as raw materials for producing new products.

A typical plastic film washing line primarily includes the following core equipment:

1. Shredder or Crusher
2. High-Speed Friction Washer
3. Floating Tank
4. Dewatering Machine/Drying System

Materials are transferred between equipment via screw conveyors or pneumatic conveyance. While there are various types of washing equipment, the basic principles are similar. To improve product quality, auxiliary equipment such as trommel screens, wind sifters, and bounce separators can be added to remove contaminants.

Pre-treatment: Shredding and Crushing

The first step in the washing line is to break the film into small pieces, addressing several issues:

• Preventing whole plastic bags from wrapping around equipment, affecting output and safety
• Dispersing adhered films (such as cling film and LLDPE film)
• Facilitating material flow between equipment

Wet grinders or industrial shredders are typically used for this crushing process.

Shredders

Shredders are heavy-duty, low-speed, high-torque equipment. Cutting blades are mounted on slowly rotating shafts, capable of tearing large materials into irregular 1-5 inch pieces. The low-speed operation ensures minimal or no dust generation.

Shredders can be classified by the number of shafts: single-shaft, double-shaft, and four-shaft types. Besides the plastics industry, shredders are widely used in:

• Rubber recycling
• Metal recycling
• Electronic waste processing
• Wood processing
• Sensitive item destruction (e.g., expired food, medicines)
• Wastewater treatment plants and other government applications
• Solid waste pre-treatment (e.g., food, pallets, plastic drums, furniture)

In fully automated plastic washing lines, shredders are particularly useful. They can process whole bales of film, which is important for recycling centers aiming to save space and reduce logistics costs. The size of plastic film bales varies depending on the baling machine and operator, typically 3-5 feet on each side, weighing 500-1,500 pounds.

Crushers

Unlike shredders, crushers operate at higher speeds, typically 200-800 rpm. Heavy-duty angled blades are mounted on the shaft. When the rotating knives contact the fixed blades in the crushing chamber, the material is cut into smaller fragments. A screen with customizable apertures at the bottom or around the shaft ensures that material only passes through when it reaches the desired size.

While crushers can produce smaller output, their high-speed operation generates more noise and dust. High-quality crusher manufacturers usually provide soundproof enclosures and dust reduction measures, such as additional screens, speed reduction, pneumatic conveyance, and wet crushing.

For plastic film washing, wet grinders are usually recommended. They continuously spray water into the crushing chamber during the process, not only completing the crushing but also pre-washing dirty films. The water also acts as a lubricant, reducing heat and wear, thus extending blade life.

Comparison of Shredders and Crushers

Choosing the appropriate crushing equipment requires weighing their pros and cons. Some washing lines even use both types of equipment: a shredder for pre-treating large film pieces to palm-size, followed by a wet crusher for further breaking down and pre-washing.

Shredder Advantages and Disadvantages:

Advantages:

• Can process whole film bales, reducing manual labor
• Low-speed operation, less noise, less dust
• Suitable for processing low-quality films with high contaminant content
• Some models offer wet shredding functionality

Disadvantages:

• Output fragments are uneven in size and shape, relatively large
• Difficult maintenance, time-consuming blade replacement

Crusher Advantages and Disadvantages:

Advantages:

• Produces smaller, more uniform fragments
• Easy maintenance, convenient blade replacement

Disadvantages:

• Requires manual separation of film bales before feeding
• High-speed operation, noisy, dusty
• Easily damaged by hard objects, not suitable for heavily contaminated materials

High-Speed Friction Washer

While not essential, high-speed friction washers are very useful in washing lines. Their core is a long shaft with high-speed rotating blades. Around the shaft is a screen channel for dewatering. The entire device is enclosed in a rectangular housing with water pipes spraying onto the screen. The equipment is inclined, with material fed from the lower end and cleaned material exiting from the upper end.

High-speed friction washers are typically placed after the wet crusher. The mixture of dirty water and contaminants enters the washer, which rotates at over 1000 rpm. The rotating blades strike the film fragments at high speed, creating intense friction. Centrifugal force throws contaminants out through the screen. Continuous water spraying prevents screen clogging and replenishes water in the washer.

For materials containing small amounts of paper contaminants, high-speed friction washers are particularly effective, as the intense friction can break down paper into pulp and expel it through the screen.

The inclined angle of the blades moves material upward, finally exiting from the top into a floating tank or more advanced liquid-solid separator. Some processing plants may choose to use long water tanks or floating tanks instead of high-speed friction washers to extend soaking time for better separation of contaminants.

Floating Tank

Floating tanks (also known as sink-float tanks) have two main functions:

1. Cleaning: Soaking films to facilitate contaminant separation
2. Separation: Using density differences to float light films and sink heavy impurities

A typical floating tank is a large rectangular water tank, about 2-3 meters wide, 4-6 meters long, and 2-3 meters high. As plastic film enters the tank, it’s moved forward by slowly rotating paddles or water flow. Due to density differences, plastic film (density <1 g/cm³) floats on the surface and moves forward, while impurities denser than water (like stones, sand) sink to the bottom.

The bottom of the separation tank is conical, and collected waste is removed via a conveyor belt controlled by pneumatic valves. This waste, mainly containing water, can be incinerated or sent to specialized landfills.

Cyclone Separator

Cyclone separators, commonly used in PET bottle recycling, are also suitable for plastic film washing. They work similarly to floating tanks but with better separation efficiency, generating centrifugal force about 20 times that of Earth’s gravity. In the cyclone separation process, air pressure is much higher than in tank mixing devices. The centrifugal force generated by the cyclone causes light films to rotate outward and upward, while heavy materials sink.

Drying System

Drying is the final step in the film washing line and a crucial process before extrusion pelletizing. Thorough drying prevents air pockets during extrusion; film moisture content should be controlled below 15%. Two common drying methods are:

1. Dewatering Machine (Centrifuge) + Hot Air Drying:
   The dewatering machine uses centrifugal force to remove most moisture, reducing film moisture content to 20-30%. Hot air further dries it to about 15%.
2. Screw Press:
   Economical method that removes water by squeezing and twisting the film.

Dewatering Machine/Hot Air Dryer

Dewatering machines are similar in structure to high-speed friction washers, with a central high-speed rotating (about 1000 rpm) blade shaft surrounded by a screen. As wet film enters, centrifugal force throws water out through the screen.

Hot air drying involves mixing dewatered film with hot air, rotating it in a conveying channel, and finally passing it through a cyclone separator to further evaporate remaining moisture. Typically, 1-3 stages of hot air drying are used, depending on requirements.

Screw Press

Screw presses are widely used in food production, papermaking, wastewater treatment, and other industries. They effectively dewater without requiring expensive hot air drying equipment.

The core of a screw press is a spiral shaft with gradually increasing diameter, surrounded by a perforated drainage channel. As wet film enters, it’s compressed as the spiral shaft diameter increases, forcing water out through small holes. The narrow gap at the end of the equipment can reduce moisture content to about 15%.

Customizing a Film Washing Line

The specific configuration of a washing line depends on the type of film to be processed and its level of contamination. For example, shredders are more suitable than crushers when dealing with heavily contaminated materials or high production volume requirements.

A typical LDPE film (98/2 or Grade A film) washing line might include: wet crusher, high-speed friction washer, floating tank, centrifugal dewatering machine, and hot air dryer. Alternatively, the water tank could be extended, eliminating the need for a high-speed friction washer. Some factories use two high-speed friction washers, one before and one after the washing tank. To reduce crusher wear, a shredder can be added for pre-treatment.

More equipment results in cleaner products but also increases investment and operating costs. In areas with lower labor costs, manual labor can replace some equipment, such as using manual transfer between equipment instead of conveyors.

Conclusion

This article introduces typical equipment commonly used in plastic film washing lines. There’s no fixed standard for recycling methods; the key is to choose carefully based on film type and contamination level. Given the significant performance differences between equipment, it’s advisable to visit manufacturer factories before making decisions.